The present invention relates to the field of lithographic printers used in patterning radiation sensitive layers which are used as stencils, molds, or etch control devices and the products made from such devices. More specifically the invention relates to a process for fabricating a mask with an attenuating layer where the thickness of the layer varies discreetly or in a continuous manner.
Production of diffractive optical elements (DOE), computer generated holograms (CGH), and kinoform optics (KO) has previously been accomplished through multiple lithographic processing steps. A multistep method is not optimal because it is time consuming, complex, and because every step in the process creates an opportunity to introduce errors into the process (e.g. misalignment between lithographic steps). FIGS. 1a and 1f demonstrate a prior art multi-binary mask approach to fabricating a micro-optics using multiple lithographic masks.
In response to the recognized deficiencies of the multi-step process, there has been a drive to develop a single step high resolution (0.1 to 1 micron) lithographic mask (with gray scale) and etch process for the production of such optical elements. Prior art approaches to developing gray scale attenuating masks have involved half tone, chrome on glass or variable e-beam exposure of ion implanted glass. To date, resolution due to e-beam scatter and lack of sensitivity to e-beam writing makes single-step gray scale masks very expensive to produce and of limited applicability. This lack of sensitivity requires extensive exposure time, and increases the manufacturing cost. Masks produced by this method currently cost many times that of binary masks produced by methods well known to practitioners skilled in the art. Further, with ion implanted glass the optical density of the material is limited by the density and thickness of the implanted layer (normally 3 to 10 microns), while the substrate material of glass is not transmissive below xcx9c350 nm wavelength, which limits its application to the longer wavelength lithographic systems.
The present invention overcomes the limitations noted above and creates a lithographic mask that complies with the desired single step lithographic process for fabrication of the diffractive optical devices mentioned.
A mask and method of fabricating a mask for use in patterning a radiation sensitive layer in a lithographic printer is provided. The method requires providing a substrate and coating the substrate with a layer of radiation attenuating material. The attenuating layer may have phase shifting characteristics. The layer of radiation attenuating material is then coated with a layer of radiation sensitive material, which is exposed to radiation having an intensity or amplitude varying on an analog scale from a radiation source. After exposure, the layer of radiation sensitive material is developed to reveal a surface relief profile. The surface profile is transferred in the layer of radiation sensitive material into the layer of attenuating material by an anisotropic etch.
Utilizing the mask formed as described above, micro optics, micro-electrical mechanical systems and integrated circuits can be formed wherein these devices comprise a substrate having a surface with smoothly varying spatial contours having a pattern replicated from a mask with a spatially varying surface profile etched by an anisotropic etch.